In recent years, the notion of mounting large lithium-ion secondary batteries, having high energy density and excellent output characteristics, in electric vehicles has been investigated in response to increasing concern over environmental issues. In small mobile device applications such as mobile telephones or laptop computers, the capacity per unit volume is important, so graphitic materials with a large density have primarily been used as negative electrode active materials. However, since lithium-ion secondary batteries for vehicles are large and expensive and thus it is difficult to replace the lithium-ion secondary batteries while using the vehicle, the lithium-ion secondary batteries for vehicles are required to have at least the same durability as that of vehicles and demanded to have a product life of 10 years or longer (high durability).
When graphitic materials or carbonaceous materials with a developed graphite structure are used, there is a tendency for damage to occur due to crystal expansion and contraction caused by repeated lithium doping and de-doping, which diminishes the charge/discharge repetition performance. Therefore, such materials are not suitable as negative electrode materials for lithium-ion secondary batteries for vehicles which require high cycle durability.
In contrast, non-graphitizable carbon is suitable for use in automobile applications from the perspective of involving little particle expansion and contraction due to lithium doping and de-doping and having high cycle durability (Patent Document 1). Therefore, there has been an attempt to use graphitic material and non-graphitizable carbon by mixing as negative electrode active materials, and lithium-ion secondary batteries having excellent cycle durability has been obtained (Patent Document 2).